This invention relates in general to a method for increasing the operational efficiency of a fuel cell power plant, and deals more particularly with a method and apparatus for providing a fuel cell power plant with a reactant stream which increases the utilization of the reactant stream, thereby increasing the performance of the fuel cell power plant as a whole.
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a reactant fuel being provided to an anode electrode substrate and a reactant oxidant being provided to a cathode electrode substrate, generating an external current flow there-between. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Typically, electrochemical fuel cell assemblies employ a hydrogen-rich gas stream as a fuel and an oxygen-rich gas stream as an oxidant, whereby the resultant reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrode substrates formed of porous, electrically conductive sheet materialxe2x80x94typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION(trademark) and well known in the art. Catalyst layers are formed between the PEM and each electrode substrate to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly. In certain fuel cell configurations, flow field plates are disposed on either side of the anode and cathode substrates and may include channels formed therein for accommodating the respective flows of reactant fuel and oxidant. The flow field plates may be additionally adapted to include coolant channels for circulating the water coolant about the fuel cell assembly.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts at the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit connected to a load. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygen-containing oxidant gas. The reactions taking place at the anode and cathode catalyst layers may be represented by the following equations:
Anode reaction H2-- greater than 2H++2e
Cathode reaction: {fraction (1/2 )}O2+2H++2e-- greater than H2O
In practical applications, a plurality of planar fuel cell assemblies are typically arranged in a stack, commonly referred to as a cell stack assembly. The cell stack assembly may be surrounded by an electrically insulating housing that defines the various manifolds necessary for directing the flow of a hydrogen-rich fuel and an oxygen-rich oxidant to the individual fuel cell assemblies, as well as a coolant stream, in a manner well known in the art. A fuel cell power plant may typically be comprised of the cell stack assembly, reactant storage vessels, reactant control valves, reactant flow devices, coolant pumps, heat exchangers, coolant degassifiers or demineralizers, sensors for measuring reactant concentrations, temperatures, pressures, current, voltage, and a microprocessor that controls the operation of the fuel cell power plant.
As will be appreciated by one so skilled in the art, tying these differing components into a cohesive fuel cell power plant operating within specific design parameters results in a complex and oftentimes cumbersome structure.
Specifically, the operating efficiency of a fuel cell power plant is directly related to the utilization of the reactant fuel stream supplied to the fuel cell assemblies making up the fuel cell power plant. This utilization, commonly referred to as xe2x80x98hydrogen utilizationxe2x80x99, due to the use of a hydrogen-rich fuel stream in fuel cell applications, is the ratio of reactant fuel consumed at the anode electrode of the fuel cells, divided by the total quantity of reactant fuel supplied to the fuel cells multiplied by 100. While PEM fuel cell power plants are designed to come as close as possible to 100% utilization, this is practically unfeasible.
Current generation PEM fuel cells frequently use thin polymer membranes on the order of approximately 15 microns thick to maximize cell performance of approximately 1000 amps per square foot (ASF). A certain measure of the hydrogen utilized as fuel within these fuel cells will diffuse across this thin membrane from the anode electrode to the cathode electrode to react with the oxygen-rich oxidant to form water. Likewise, oxygen also tends to diffuse across this thin membrane to combine with hydrogen to form water. The combination of these effects is to reduce the available hydrogen capable of reacting at the anode electrode of the fuel cells and hence, reduces the utilization rate of the fuel cell power plant as a whole.
It is common for typical fuel cell power plants to be operated by supplying a reactant fuel to the integrated cell stack assembly using either a cascade or multiple-pass approach, wherein the individual fuel cells in the cell stack assembly are separated in two or more groups. The reactant fuel is supplied to the first group of fuel cells and then cascades to the next group and so on until exiting from the cell stack assembly through a fuel exhaust manifold. With such an arrangement, the practical hydrogen utilization has been found to be as high as 90% or more for the overall cell stack assembly, with individual cascade groups operating at approximately 60-70% utilization.
Many different flow orientations have, however, been utilized in fuel cells wherein the objective is always to distribute the hydrogen flow such that every section of the fuel cell receives the required quantity of fuel while maintaining a high hydrogen utilization rate. Known flow configurations that have been used within fuel cells are single pass, two pass, multi-passes, serpentine and interdigitated. An external recycle is oftentimes used between the cell exit and the cell inlet to improve flow uniformity within the cell stack assembly.
Regardless of the flow configurations utilized, a primary factor in maintaining a high hydrogen utilization rate resides in maintaining a proper pressure drop between the reactant fuel inlet and fuel exhaust, in conjunction with regulating the flow rate of the reactant fuel. In this respect, the multi-pass or serpentine flow configurations employ relatively narrow and elongated fuel channels operated under high flow rates to maintain the required pressure differential between the fuel input to the fuel exhaust. Similarly, the interdigitated flow configuration utilizes pairs of typically narrow fuel channels whereby fuel is provided to one of the fuel channels having a closed end and exhausted through the adjoining fuel channel after traveling through the anode substrate. In such a manner, the interdigitated flow configuration also utilizes high reactant flow rates and an extended flow path for the reactant fuel to ensure the pressure differential necessary for efficient hydrogen utilization.
It will be readily appreciated that equipping fuel cells with a fuel delivery flow field comprised of uniform, narrow, elongated fuel channels, such as those noted above, requires manufacturing tolerances which are oftentimes difficult and costly to adhere to. Moreover, plural or elongated fuel channels complicate the humidification of the fuel stream, which frequently leads to regional drying of the fuel cell and subsequent fuel cell failure.
With the forgoing problems and concerns in mind, the present invention therefore seeks to increase the hydrogen utilization for a fuel cell while enabling the use of single-pass fuel channels and relatively low reactant fuel flow rates, while concurrently reducing the complexity and demands made upon the fuel cell humidification system.
It is an object of the present invention to increase the operational efficiency of a fuel cell power plant.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant while enabling more efficient fuel cell hydration.
It is another object of the present invention to increase the hydrogen utilization of a fuel cell power plant without the need for the recycling of exhausted fuel.
It is another aspect of the present invention to increase the hydrogen utilization of a fuel cell power plant while permitting the use of single-pass fuel delivery channels.
According to one embodiment of the present invention, a method and apparatus for increasing the operational efficiency of a fuel cell power plant includes a cell stack assembly comprised of a plurality of fuel cells in electrical communication with one another, each of the fuel cells including an anode substrate in fluid communication with an anode flow field plate. The proposed method includes forming a fuel channel in the anode flow field plate, providing a fuel stream to the fuel channel and interrupting the fuel channel at a location along the fuel channel so that the fuel stream is directed to permeate the anode substrate before being exhausted from the fuel cells.
These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole.